EP2005447B1 - Cryostat having a magnet coil system, which comprises an lts section and a heatable hts section - Google Patents

Description

Background of the invention

The invention relates to a cryostat having a magnetic coil system comprising a superconductive conductor for generating a magnetic field B ₀ in a measuring volume with a plurality of radially nested, electrically connected coil sections, of which at least one LTS section comprises a conventional low temperature superconductor (LTS). and at least one HTS section comprises a high temperature superconductor (HTS), wherein the liquid helium magnetic levitation system is in a helium tank of the cryostat at a helium temperature T _L <4K.

Such a cryostat has become known, for example from the DE 10 2004 007 340 A1 ,

For example, for nuclear magnetic resonance apparatus, in particular spectrometers, very strong, homogeneous and stable magnetic fields are needed. The stronger the magnetic field, the better the signal-to-noise ratio and the spectral resolution of the NMR measurement.

To generate strong magnetic fields, superconducting magnet coil systems are used. Magnetic coil systems with solenoid-shaped coil sections, which are nested in one another and are operated in series, are widespread. Superconductors can carry electrical power without loss. The superconductivity sets itself below a material-dependent transition temperature. Conventional low-temperature superconductors (LTS) are typically used as the superconductor material. These metal alloys such as NbTi and Nb ₃ Sn are relatively easy to process and reliable in use. The conductor of an LTS coil section usually consists of a good normal conductive metallic matrix (copper), in which superconducting filaments are located, which completely take over the current during normal operation. In the case of NbTi, these are usually tens to hundreds, in the case of Nb ₃ Sn, it may be more than a hundred thousand. In fact, the inner structure of the ladder is a bit more complex, but this does not matter in the present context.

To cool the coil sections below the critical temperature, the coil sections are cooled with liquid helium in a cryostat. The superconducting coil sections dip at least partially into liquid helium.

In order to further increase the achievable magnetic field strength in a magnetic coil system, it is desirable to also use high-temperature superconductors (HTS). At the same temperature, conductors containing HTS can carry much more current and reach higher magnetic field strengths than those LTS. HTS material is therefore primarily suitable as material for the innermost coil sections of a magnetic coil system.

Currently, HTS or ceramic superconductors are mainly available as bismuth conductors with HTS filaments in a silver-containing matrix. The ladders are predominantly in the form of ribbons.

Coil sections made of HTS in supercooled helium have, however, proven to be short-lived and unreliable. An investigation of failed HTS sections revealed that the HTS material bursts and the current carrying capacity of the HTS conductor is thereby destroyed. This effect, which is also known in other contexts, is sometimes referred to as "ballooning".

Object of the invention

Accordingly, it is the object of the present invention to provide a cryostat in which a HTS coil section can be used long-term and reliable, and in particular no "ballooning" occurs.

Brief description of the invention

This object is achieved by a cryostat of the type described above, which is characterized in that heating means are provided which keep the HTS at any time at an elevated temperature T _H > T _L and T _H > 2.2 K.

The present invention is based on the finding that the "ballooning" is caused by superfluid helium, which expands or evaporates in the interior of the HTS material. As is known, helium liquefies at atmospheric pressure below about 4.2 K. Helium, however, continues to exhibit a phase transition of the second kind (λ point) at a temperature of 2.2 K. Below the λ point, liquid helium becomes superfluous, ie the helium can flow without friction and has an infinitely high thermal conductivity. Of the the former effect ensures that it penetrates even the smallest gaps, especially in spite of the sheathing through the matrix in the cavities inside a ceramic HTS. In contrast, a compression of the ceramic material helps nothing. In case of a later warming over the λ-point the helium remains trapped in the HTS. The warming also ensures an expansion of the trapped helium, especially if the warming is so strong that the helium evaporates. As a result, a considerable pressure is built up inside the HTS. Since HTS is a ceramic and thus brittle material, the HTS is finally burst locally from the pressure and degrades the conductor.

This can be prevented by the cryostat according to the invention. The HTS is maintained by the heating means according to the invention at a temperature at which superfluid helium does not occur. This ensures that no superfluid helium penetrates into the HTS. This can not come to the "ballooning".

It should be noted that the temperature T _L of most of the liquid helium in the helium tank of the cryostat can be equal to or lower than the λ-point temperature of 2.2 K according to the invention. The HTS only needs to be kept warm enough locally. A temperature T _L of 2.2 K or below is favorable even for particularly stable conditions for the LTS sections, in particular, mechanical deformations due to temperature differences are minimized. But above all, a T _L <2.2 K increases the current carrying capacity and the critical magnetic field strength in the co-cooled LTS sections.

A preferred embodiment of the cryostat according to the invention provides that the heating means keep the HTS at an elevated temperature T _H > 2.5 K at all times. Even above the λ-point temperature of 2.2 K, superfluid helium phase can briefly occur. With this embodiment, a sufficient buffer is set up against such fluctuations and the HTS is even better protected.

Particularly preferred is also an embodiment in which the HTS section forms the radially innermost section. Here are the largest magnetic field strengths, and the expensive and problematic HTS is used particularly effectively. Furthermore, this arrangement facilitates only local cooling of the HTS section.

Furthermore, an embodiment in which the cryostat has a room temperature bore surrounded by the magnetic coil system, in which the measurement volume is located, is preferred. The room temperature hole allows easy placement of a sample at room or variable temperature in the measurement volume.

A preferred development of this embodiment provides that there is a thermal contact between the innermost section and the wall of the helium tank facing the room temperature bore, wherein the contact forwards radiated heat radiation to this wall. This passive heating of the HTS section is particularly reliable, since a sufficient temperature for heating the HTS section on the design of radiation shields and the mechanical coupling to the wall of the helium tank and possibly the prevention of convection in the helium tank to the HTS section easy to ensure around. In particular, the passive heating makes a power failure for the HTS harmless.

Another advantageous embodiment provides that there is a thermal contact between the HTS section through the wall of the helium tank to a radiation shield as a heating means, wherein the radiation shield is at a temperature T _S > T _L , in particular wherein T _{S is} about 40 K. , This heating is passive and thus energy saving. Again, a power failure in particular for the HTS is safe.

Also advantageous is an embodiment in which the heating means comprise an electric heater. The electric heater is easy to control and allows accurate temperature control of the HTS section even outside normal operating conditions, especially when filling or emptying the helium tank or when quenching.

Particularly preferred is also an embodiment of the cryostat according to the invention, which provides that the HTS section and possibly also the thermal contact has a jacket for thermal insulation against the surrounding helium. This embodiment reduces the necessary cooling power for the liquid helium in the cryostat, which compensates for the heat input of the heating means. Furthermore, the HTS is also mechanically protected against superfluid helium.

In a preferred development of this embodiment, the sheath also extends to superconducting leads to the HTS section, at least as far as the leads HTS included. This also includes the joints in the protection against penetrating superfluid helium.

A further development provides that the sheath is formed of plastic, in particular by a multilayer epoxy resin.

A preferred embodiment provides that the magnetic field B ₀ generated in the measurement volume by the magnet coil system is greater than 20 T, in particular greater than 23 T. These strong magnetic fields are easily accessible by means of HTS section and the cryostat according to the invention. In contrast, with conventional magnet systems based only on LTS sections, the theoretical limit is already reached at these field strengths, and the critical current density tends toward zero.

Also preferred is an embodiment in which the coil sections of the magnet coil system can be superconductingly short-circuited during operation. As a result, for example, for NMR and ICR (Ion cyclotron resonance) required stability achieved.

Also preferred is an embodiment, which is characterized in that the magnetic coil system with respect to the homogeneity of the magnetic field B ₀ in the measurement volume and the time stability of B ₀ meets the requirements of high-resolution NMR spectroscopy, which requires a special design of the magnetic coil system and the cryostat which is known per se for pure LTS systems.

Finally, another embodiment of the cryostat according to the invention is preferred which provides that means are provided in the helium tank for minimizing convection of helium around the HTS section. These means are, for example, mechanical barriers located on or near the surface of the HTS section and obstructing the helium flows on the surface of the HTS section or on the surface of parts that are thermally coupled to the HTS section. The reduced convection reduces heat input by the heating means into the liquid helium, and conversely reduces the cooling efficiency of the liquid helium at the HTS section. As a result, the cryostat is more economical and stable to operate.

Further advantages of the invention will become apparent from the description and the drawings. Likewise, according to the invention, the above-mentioned features and those which are still further developed can each be used individually for themselves or for a plurality of combinations of any kind. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

Fig. 1

eine erste Ausführungsform eines erfindungsgemäßen Kryostaten mit thermischem Kontakt der HTS-Sektion zur Wand des Heliumtanks, die der Raumtemperaturbohrung zugewandt ist, in schematischer Darstellung;

Fig. 2

eine zweite Ausführungsform ähnlich Fig. 1, mit zusätzlichen wärmeleitenden Kontaktfedern zum Strahlungsschild im Bereich der Raumtemperaturbohrung;

Fig. 3

eine dritte Ausführungsform eines erfindungsgemäßen Kryostaten, mit einem thermischen Kontakt der HTS-Sektion zum Strahlungsschild im Bereich des Bodens in schematischer Darstellung;

Fig. 4

eine vierte Ausführungsform eines erfindungsgemäßen Kryostaten mit einer elektrischen Heizung in schematischer Darstellung.

The invention is illustrated in the drawing and will be explained in more detail with reference to embodiments. Show it:

Fig. 1

a first embodiment of a cryostat according to the invention with thermal contact of the HTS section to the wall of the helium tank, which faces the room temperature bore, in a schematic representation;

Fig. 2

a second embodiment similar Fig. 1 , with additional heat-conducting contact springs to the radiation shield in the range of the room temperature hole;

Fig. 3

a third embodiment of a cryostat according to the invention, with a thermal contact of the HTS section to the radiation shield in the region of the bottom in a schematic representation;

Fig. 4

A fourth embodiment of a cryostat according to the invention with an electric heater in a schematic representation.

The FIG. 1 schematically shows a first embodiment of a cryostat 1 according to the invention. The cryostat 1 has a room temperature bore 2, in which an examination volume 3 is provided for a sample. The examination volume is located in the center of a magnetic coil system which is formed from here three solenoid-shaped coil sections 4, 5, 6. The magnet coil system generates a homogeneous magnetic field B ₀ in the examination volume 3. The radially innermost coil section 4 is wound with a wire of high-temperature superconductor (= HTS). The middle coil section 5 is wound with Nb ₃ Sn wire and the outermost coil section 6 is wound with NbTi wire. The coil sections 5, 6 thus represent low-temperature superconductor (= LTS) coil sections. The coil sections 4, 5, 6 are electrically connected in series with one another, by way of example with the two superconducting transition points (joints) 7a and 7b. At the joint 7a, the HTS material of the HTS section 4 is connected to an NbTi transition piece 8, and at the joint 7b, the transition piece 8 is connected to the Nb ₃ Sn wire of the LTS section 5.

The coil sections 4, 5, 6 are located inside a helium tank 9, which is largely filled with liquid helium. The liquid helium in the helium tank 9 has a temperature T _L of less than 4 K, for example about 2.0 K. The helium in the helium tank 9 is constantly cooled by a cooling device, not shown, to compensate for heat input from the outside and to keep T _L constant , see eg US 5,220,800 , Alternatively to the design according to Fig. 1 can the helium tank like in the US 5,220,800 have two chambers separated by a thermal barrier, which are at temperatures of about 2 K and 4 K, wherein the magnetic coil system is disposed in the 2 K chamber.

The LTS coil sections 5, 6 located in this helium bath have likewise assumed the temperature T _L. The situation is different with the HTS coil section 4. This has a thermal contact 10, which connects the HTS section 4 with the wall 11 of the helium tank 9, which faces the room temperature bore 2 (and the examination volume 3), heat-conducting. Thermal radiation which impinges on the wall 11 then provides heat input via the thermal contact 10 into the HTS section 4. This heat radiation can be emitted, for example, from the radiation shield 12 surrounding the helium tank 9. The radiation shield 12 in particular undergoes heat radiation from the wall of the room temperature bore 2. The radiation shield 12 has a temperature of approximately 40 K.

In equilibrium between heat input via the thermal contact 10 and the cooling by the surrounding liquid helium, a temperature T _H , which is greater than T _L , and according to the invention also greater than the temperature of the λ point of FIG. ^4, arises at the HTS section ⁴ He is about 2.2K. For example, T _{H is} about 3.0 K. This value of T _H is sufficient to prevent ingress of superfluid helium into the HTS section and into the HTS itself, ie helium remains in the area of the surface of the HTS section 4 normal liquid and can not penetrate deeper.

The FIG. 2 shows a slightly modified embodiment of the cryostat 1. If the heat input by radiant heat to the wall 11 and thus in the thermal contact 10 should not be sufficient to sufficiently heat the HTS section 4, contact fields 21 may be provided. These contact springs 21 connect a relatively warm part of the cryostat 1 (warmer than T _L and warmer than 2.2 K), namely here the radiation shield 12 (with T _S about 40 K), with the wall eleventh

The FIG. 3 shows a third embodiment of a cryostat 1 according to the invention. The HTS section 4 is connected to another thermal contact 31. This thermal contact 31 is guided through the bottom 32 of the helium tank 9 and connected to the radiation shield 12 in the bottom region thereof. The radiation shield 12 has a temperature T _S of about 40 K and thus can deliver sufficient heat to the HTS section 4 to prevent ingress of superfluid helium in the HTS section 4. The heat input can be easily adjusted, for example, via the diameter of the thermal contact 31. The thermal contact 31 is preferably thermally insulated over its entire length to the ends, for example by a plastic jacket.

Furthermore, means 33 are provided at the upper edge of the HTS section 4, which prevent helium between the thermal contact 31 and the wall 11 of the helium tank 9 can flow. The means 33 are annular for this purpose. Alternatively, the function of the means can also be taken over by the thermal contact 31 itself, or the HTS section 4 is sufficiently close to the wall 11, so that no convection can occur.

The FIG. 4 Finally, FIG. 4 shows a fourth embodiment of a cryostat 1 according to the invention. The HTS section 4 is not actively (only) actively heated via thermal contacts but via an electric heater 41. For this purpose run heating coil (for example made of copper) on the surface or inside the HTS section 4. The heating power is chosen so that the desired temperature T _{H of} the HTS section 4 results. According to the invention, a temperature sensor may be provided on or in the HTS section 4 in order to control T _H. In general, a constant heating current is used. For simplicity, the electrical leads and the power supply of the electric heater 41 are not shown.

The HTS section 4 is additionally surrounded by a sheath 42 of three-layered epoxy resin, which thermally isolates the HTS section 4 from the surrounding liquid helium and also mechanically separates it. The jacket 42 also includes the joint 7a, so that all HTS material is enclosed by the jacket 42. In the area of the joint 7a, an additional heating coil is provided.

The cryostat 1 of the FIGS. 1 to 4 are preferably parts of an NMR apparatus, such as an NMR spectrometer or an NMR tomograph, in particular a high-resolution high-field NMR spectrometer with a magnetic field B ₀ > 20 T, preferably> 23 T in the measurement volume, the magnetic coil system with respect to the homogeneity the magnetic field B ₀ in the measurement volume and the time stability of B ₀ meets the requirements of high-resolution NMR spectroscopy, which usually requires that the coil sections of the magnetic coil system can be superconductingly short-circuited during operation.

Claims (14)

1. Cryostat (1),
   having a magnet coil system which comprises superconductors, for production of a magnet field B₀ in a measuring volume (3), with a plurality of radially nested solenoid-shaped coil sections (4, 5, 6) which are electrically connected in series, at least one section of which being an LTS section (5, 6) having a conventional low temperature superconductor (LTS) and with at least one section of which being a HTS section (4) having a high temperature superconductor (HTS), wherein the magnet coil system is disposed with liquid helium in a helium tank (9) of the cryostat (1) at a helium temperature T_L < 4 K,
   characterized in that
   heating means are provided which keep the HTS at an increased temperature T_H > T_L and T_H > 2.2 K at all times.
2. Cryostat (1) according to claim 1, characterized in that the heating means keep the HTS at an increased temperature of T_H > 2.5 K at all times.
3. Cryostat (1) according to any one of the preceding claims, characterized in that the HTS section (4) forms the radially innermost section (4).
4. Cryostat (1) according to any one of the preceding claims, characterized in that the cryostat (1) has a room temperature bore (2) surrounded by the magnet coil system, wherein the measuring volume (3) is located within the room temperature bore (2).
5. Cryostat according to claims 3 and 4, characterized in that a thermal contact (10) between the innermost section (4) and the wall (11) of the helium tank (9) facing the room temperature bore (2) functions as the heating means, wherein the contact (10) passes on radiative heat irradiated onto the wall (11).
6. Cryostat (1) according to any one of the preceding claims, characterized in that a thermal contact (31) between the HTS section (4) and a radiation shield (12) functions as the heating means, wherein the radiation shield (12) is at a temperature T_S > T_L, in particular wherein T_S is approximately 40 K.
7. Cryostat (1) according to any one of the preceding claims, characterized in that the heating means includes an electrical heater (41).
8. Cryostat (1) according to any one of the preceding claims, characterized in that the HTS section (4) and, if appropriate also the thermal contacts (10, 31), have a jacket (42) for thermal insulation with respect to the surrounding helium.
9. Cryostat (1) according to claim 8, characterized in that the jacket (42) also extends to the superconducting leads of the HTS section (4), at least to the extent that these leads contain HTS.
10. Cryostat (1) according to one of the claims 8 or 9, characterized in that the jacket (42) is made from plastic material, in particular from a multi-layer epoxy resin.
11. Cryostat (1) according to one of the preceding claims, characterized in that the magnet coil system produces a magnetic field (B₀) in the measuring volume (3), which is larger than 20 T, in particular larger than 23 T.
12. Cryostat (1) according to any one of the preceding claims, characterized in that the coil sections (4, 5, 6) of the magnet coil system may be superconductively short-circuited during operation.
13. Cryostat (1) according to claim 12, characterized in that the magnet coil system fulfills the requirements of high resolution NMR spectroscopy with regard to the homogeneity of the magnetic field B₀ in the measuring volume as well as the temporal stability of B₀.
14. Cryostat (1) according to any one of the preceding claims, characterized in that means (33) are provided in the helium tank (9) which minimize convection of helium about the HTS section (4).

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